1. Technical Field
This invention relates to formable surgical fasteners and, more particularly, to directionally biased formable staples for use in surgical staplers having anvil pockets for forming the staples.
2. Background of Related Art
Surgical stapling instruments have become critical to many life saving surgical procedures. Surgical staples are usually mechanically inserted into tissue with surgical stapling instruments such as those known as anastomosis devices, including gastrointestinal anastomosis devices and transverse anastomosis devices. In such devices, the staples are loaded in one or more elongated rows into a cartridge. A mechanism for pushing, or driving the stapler is actuated to drive the staples through two or more sections of tissue toward a deforming anvil. At the conclusion of the driving operation, the legs of each staple are conventionally clamped or bent, by the anvil, to a closed configuration to complete the suture and join the tissue sections together. Gastrointestinal anastomosis-type devices drive and bend the staples aligned in a row sequentially in rapid sequence, while transverse anastomosis-type devices drive and bend all staples simultaneously. See, e.g. U.S. Pat. Nos. 4,520,817 and 4,383,634. Circular anastomosis-type devices simultaneously apply annular rows of staples to tissue. See, e.g. U.S. Pat. No. 4,304,236.
One type of conventional staple 20, shown in FIGS. 1-3, used with both gastrointestinal anastomosis and transverse anastomosis-type surgical stapling devices is made of stainless steel or titanium. The undeformed staple 20 (FIG. 1) is generally U-shaped and includes a back span 22 and two legs 24 depending substantially perpendicularly from the back span. Each leg 24 has a sharp chiseled end point 26 for piercing body organs or tissue. The chisel point also creates torque in the staple, allowing it to form. The staple penetrates the tissue from one side to engage an anvil spaced apart and located at an opposing side of the tissue. The staple is bent by having the legs engage and follow an anvil 25 to form a B-shaped closed staple 28 as shown in FIG. 2. In this closed configuration tissue is compressed between the legs and backspan of the staple.
Because of their substantially circular cross-section (FIG. 3), these conventional staples require approximately the same amount of force to form the staple into its final shape as is required to twist or malform it.
For example, referring back to FIG. 3, a conventional round cross section staple has a moment of inertia in the x forming dimension (Ix) given by the equation:Ix=1/4πr4 Its moment of inertia in the y twisting dimension (Iy) is given by the same equationIy=1/4πr4 
Using a round wire stock of uniform 0.009 in diameter (r=0.0045),
      I    x    =            I      y        =                            1          4                ⁢                              π            ⁡                          (              .0045              )                                4                    ⁢                          ⁢                          =              3.22        ×                  10                      -            10                          ⁢                                  ⁢                  in          4                    
The Moment of Inertia Ratio, given by the equation:is Iy/Ix 
            3.22      ×              10                  -          10                    ⁢                          ⁢              in        4                    3.22      ×              10                  -          10                    ⁢                          ⁢              in        4              =  1In order to insure accurate and consistent formation of these conventional staples, considerable research and development has been conducted in the areas of forming and driving structures. For example, anvils have been developed with specific coatings and/or structure, see, e.g. U.S. Pat. Nos. 5,173,133 and 5,480,089. Also, staple cartridges have been configured with driver structure to balance forces encountered during staple formation. See, commonly assigned U.S. Pat. No. 4,978,049 to Green. Thus, to control and insure consistent staple formation without twisting or deformation, extremely strict manufacturing tolerances have been implemented.
Other types of staples for different types of instruments are also found in the prior art. Some have non-circular cross-section. FIGS. 4, 4A and 4B illustrate by way of example a staple of this type marketed by United States Surgical of Norwalk, Conn. for use with its MULTIFIRE ENDO HERNIA and ENDO UNIVERSAL 65 staplers. The anvil in these staplers, as shown in FIGS. 4C and 4D, is adjacent the backspan of the staple as tissue is approached from only one side. Unlike the staples described above which are formed by contact of the staple legs with anvil pockets, these staple legs are bent around an anvil abutting the backspan. This staple has a side portion H with a height dimension greater than the dimension of the base portion B (i.e. 0.020 in vs. 0.015 in.).
The Moment of Inertia Ratio is given by the equation:
      Moment    ⁢                  ⁢    of    ⁢                  ⁢    Inertia    ⁢                  ⁢    Ratio    =                    I        y            Ix        =                  Moment        ⁢                                  ⁢        of        ⁢                                  ⁢        Inertia        ⁢                                  ⁢        About        ⁢                                  ⁢        Twisting        ⁢                                  ⁢        Axis                    Moment        ⁢                                  ⁢        of        ⁢                                  ⁢        Inertia        ⁢                                  ⁢        About        ⁢                                  ⁢        Forming        ⁢                                  ⁢        Axis            where Ix=(1/12)bh3 and Iy=(1/12)hb3, with h=0.020 in. and b=0.015 in.
Thus, Ix=(1/12)(0.015)(0.020)3=1.0×10−8 in4, and
Iy=(1/12)(0.020)(0.015)3=6.0×10−9 in4.
Accordingly,
      Moment    ⁢                  ⁢    of    ⁢                  ⁢    Inertia    ⁢                  ⁢    Ratio    =                    6.01        ×                  10                      -            9                          ⁢                                  ⁢                  in          4                            1.10        ×                  10                      -            8                          ⁢                                  ⁢                  in          4                      =                  .60        /        1            =      .60      
This staple is specifically configured to accommodate twisting during staple formation to permit the legs of the staple to cross as shown in FIG. 4E. Thus, it is engineered so the force to form the staple is slightly greater than the force to malform or twist the staple. The forming is accomplished by bending the staple legs around an anvil positioned adjacent the inner surface 32 of the backspan 34.
U.S. Pat. No. 5,366,479 describes a hernia staple with adjacent anvil having a height of 0.38 mm and a thickness of 0.51 mm. This staple is formed the same way as in FIGS. 4C and 4D. The moment of inertia ratio of this staple in accordance with the foregoing formula is as follows:Ix=(1/12)(0.51)(0.38)3=2.33×10−3 Iy=(1/12)(0.38)(0.51)3=4.2×10−3 
      Moment    ⁢                  ⁢    of    ⁢                  ⁢    Inertia    ⁢                  ⁢    Ratio    =                    4.2        ×                  10                      -            3                                      2.33        ×                  10                      -            3                                =    1.8  
This staple for use as described would actually result in greater force to produce the desired shape. In fact, the staple legs would likely contact each other before crossing over into their crossed configuration.
Thus, it is apparent that this type of hernia staple, i.e. where the anvil is adjacent the backspan as the tissue is approached from only one side, is quite different than the staple of the present invention, e.g. the B-shaped staple, wherein the legs penetrate through the tissue to contact anvil pockets. These anvil pockets direct the staple legs to form the staple into a closed configuration. Thus staple configuration and considerations of twisting, bending and staple formation of these hernia staples are inapplicable to these considerations for anvil pocket directed staples, such as the B-shaped staples.
It would therefore be desirable to provide a staple configuration for a staple designed to penetrate tissue and contact an anvil pocket on the opposing side of tissue, which, in complement with conventional cartridge and anvil technology, enhances correct staple formation while reducing twisting/malformation caused by misalignment or unusual tissue while minimizing reliance on strict manufacturing tolerances.